ZIP to ZIP — the same container, rebuilt with a different compression effort
This route is not a format change: a ZIP goes in and a ZIP comes out. What changes is how the member bytes are stored. Some export tools write members uncompressed, others pick a weak setting; a bundle of plain text, JSON or source files often shrinks when it is recompressed. Unsupported or unsafe entries stop the whole operation: this is not a cleaning pass that silently removes them.
Members are deflated again; original stored sizes and compression settings are not reproduced.
Processed on your device. Files aren’t sent anywhere.
Recompression is a pass over the same container
Each member is streamed through native decompression, its CRC checked, and then written again into a new ZIP using the deflate level you chose. Expansion is bounded while it streams, so the pass never materialises an enormous tree in memory.
- Input and output are both ZIP — this is a utility pass, not a new format.
- Members are deflated again; the original stored sizes and settings are not reproduced.
- CRCs are validated on read, so a corrupt member stops the pass rather than producing a shortened copy.
- The new archive keeps archive order; sorted-by-name puts the directory in alphabetical order instead.
The three levels and what they change
Only three deflate levels are exposed, because the choice is about effort rather than about exotic settings.
| Level | Behaviour | Typical use |
|---|---|---|
| 1 | fastest deflate, least size reduction | a bundle repacked many times during a session |
| 6 | default, balanced effort | ordinary delivery bundles and backups |
| 9 | maximum deflate effort | text-heavy archives where size matters more than speed |
Why a level-9 pass can gain almost nothing
If members are already compressed — images, audio, videos — there is little redundancy left to remove, so the rebuilt archive may be nearly the same size. Recompression helps most on archives dominated by text, JSON and source files.
The level affects only the stored payload of each regular file. Names, directory entries and entry order are independent of it, so switching from 6 to 9 does not rename or reorder anything.
Recompression questions
Is ZIP to ZIP a new format?
Will level 9 always make the archive smaller?
Does the rebuilt archive keep the original dates?
Is anything uploaded?
What the pass keeps and what it drops
The rebuilt archive is a fresh container, so metadata the original carried is not reproduced. Regular file names and payload bytes survive exactly, and directories are kept so the folder shape is unchanged.
A recompressed archive is a new container holding the same contents, not the same container edited in place.
The distinction that matters when you compare before and after
Running the pass
- Load the .zip into the archive studio; a single file, no larger than 12 MiB.
- Choose the deflate level; 6 is the default.
- Choose entry order if the delivery expects a specific sequence.
- Wait for the pass to complete — the reader accepts up to 200 entries and 8 MiB for any single member.
- If the pass rejects, inspect the file-list reason. Unsupported file types, encoding, archive variants, unsafe paths, damaged checksums or size limits stop the whole operation rather than being removed from the rebuilt copy.
- Download the rebuilt ZIP and compare its entry list with the original.